ERF European Rotorcraft Forum
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Stability analysis of whirl flutter in a rotor-nacelle system with freeplay nonlinearity
Get PDFTiltrotor aircraft are growing in importance because of their unique flight envelope. However, aeroelastic stability - particularly whirl flutter stability - is a major design influence that requires accurate prediction. Research efforts to make future tiltrotor aircraft larger and faster result in more difficult prediction of whirl flutter onset. Additionally, several nonlinearities that may be present, such as freeplay, are often neglected in analyses for simplicity, or they are investigated using stability analysis methods that do not capture their effects. However these nonlinearities can be significant, sometimes even reversing the stability predictions from linear analysis methods. This paper investigates the effect of a freeplay nonlinearity in the pitch degree of freedom of two rotor-nacelle models of contrasting complexity. The modelling approach and the stability analysis methods employed are explained. Ultimately the freeplay nonlinearity is shown to have a complex effect on the systems’ dynamics, including creating the possibility of whirl flutter in parameter ranges that linear analysis methods predict to be stable. This effect is demonstrated via a comparison of stability boundaries for the linear and freeplay versions of the basic model
Dynamic modeling and simulation of a cable/tow body system of a helicopter
No full textFor cases involving a helicopter involving a towing cable with an end mass, when the mass of the towed body is of the same order of magnitude of the towing cable mass, the dynamic analysis and simulation of the resulting system can become quite challenging, especially in cases that involve large cable lengths. This paper develops an approach utilizing the natural modes of a hanging string with tip mass in free vibration. With a modal representation of the cable deflection, the problem solution reduces to response to applied aerodynamic forces on the cable and the towed body. The aerodynamic forces acting on the cable are calculated using the cross-flow principle and the aerodynamic forces on the towed body are included using a table look up variations with angle of attack and sideslip angles, which are pre-computed using a comprehensive CFD analysis. The resulting equations of motion in generalized coordinates are numerically integrated for determining the cable transient deflections due to helicopter maneuvers. Results from transient simulation of the cable/tow body system for selected cases of helicopter maneuvers are presented. Further, the impact of variations of cable/tow body system parameters on the tow body transient motion are analyzed
The development of prop-rotor system for 52kg MTOW Quad-Tilt Prop (QTP) UAV
Get PDFElectric Drone (UAV) was introduced to public people world widely. Especially the quadcopter which is one type of multi-copters was very popular in people who enjoy personal hobby and leisure. The market required to improve payload and forward flight speed. From this worldwide market trend, KARI had started to pay attention to electric drone (UAV) market such as the high-speed drone which has a capability of vertical takeoff and landing (VTOL). Specially, the drone for observation and reconnaissance purpose gives an impressive attention to government officers, industry and military. For these purposes, KARI decided to develop 52kg MTOW Quad-Tilt Prop UAV (QTP UAV) based on KARI’s existing tilt-rotor technologies. This paper introduces the development of 52kg MTOW QTP UAV and its Prop Rotor system. KARI had experienced rotor system development for over 20 years. KARI had developed the optimized propeller and hub system for this QTP UAV applying optimization of the hover performance and high forward flight speed performance. To develop this prop-rotor system, new propeller’s aerodynamic design and structural design with carbon composite and titanium materials was conducted. For these 3 years development period, KARI had setup the development process of the prop-rotor system with high performance for QTP UAV based on helicopter rotor system’s experience and optimization process
Pilot model development and human manual control considerations for helicopter hover displays
Get PDFHead-down hover displays and instrument panels theoretically provide all necessary flight data information to control low-speed helicopter manoeuvring. However, past experiments have shown that head-down displays can incur high workload, control instability, and even loss of control when used as the sole flight data source. This paper investigates the reasons for this instability incurred by replacing good outside visuals with a head-down hover display and an instrument panel. A pilot model based on crossover theory is developed for a linear six-degree-of-freedom B0105 helicopter model. Utilising a target trajectory based ?-theory and assuming perfect information availability, the developed model can perform the required manoeuvring task with a control time-delay stability margin of 0,15s (with SAS) or 0,17s (without SAS). Then, the actual information availability based on human perception methods and limitations is discussed. A pilot-in-the-loop experiment in the SIMONA Research Simulator qualitatively validates the developed pilot model for good outside visuals. However, the pilot model does not capture the added difficulties of having to utilise the hover display and instrument panel instead of good outside visuals; during the experiment, the task was impossible to complete with only these displays. This is likely caused by an increase in control time-delay, which in turn is caused by the loss of peripheral and flow field information, a more abstract information representation compared to good outside visuals, and the fact that the pilot now needs to scan multiple displays to acquire all necessary flight state information. Improving head-down hover display symbology and scaling factors might rectify some, but probably not all of these effects
Computational and experimental study of a helicopter torsion rotor
No full textA computational and experimental study of a helicopter torsion rotor is presented. The motion and deformation of the blade of a three-blade main helicopter rotor model were experimentally investi-gated. Measurements were carried out using a non-contact videogrammetry method using only one digital camera. The experimental data were approximated by Fourier series. The flow around the rotor under experiment was simulated based on the unsteady vortex theory. In the calculations, rotor flapping relative to the horizontal hinge was considered. The difference in the flapping characteristics in case of hinged and torsion rotors is shown. The results of numerical simulation of the flow around such a rotor as part of a coaxial system with different initial azimuths of the upper rotor blade relative to that of the lower rotor blade are presented. The effect of the initial mutual arrangement of the blades on the change in the rotor thrust coefficient and the thrust pulsation of a coaxial helicopter rotor is shown
Flight simulator testing to enhance comprehension and modeling of rotorcraft pilot couplings
Get PDFThe results of a pilot-in-the-loop flight simulator test campaign, aimed at enhancing the comprehension of the interaction between the pilot biomechanical response and the rotorcraft dynamics, are presented. Biomechanical properties - upper limbs motion and electromiographic activities of the most involved muscles - of a test pilot involved in the complex task of ship deck landing, in varying sea conditions and with different helicopter configurations, have been measured. The analysis of the collected data highlights the dependence of the muscular activity on the perceived workload ratings (Bedford scale), and with the approaching of the most challenging portion of the simulated mission, namely the moving deck landing. Several fallbacks on numerical multibody modeling of the biomechanical behavior of the rotorcraft pilot are discussed, in the view of enhancing the quality of prediction of rotorcraft-pilot coupling phenomena
The myth of losing tail rotor effectiveness
Get PDFThe helicopter community has been plagued during the latest forty years by accidents due to unanticipated yaw, also called Loss of Tail rotor Effectiveness (LTE). How the problem was identified and what answers were given are first reconstructed from period documents and Airbus experience. A part of the mystery still remained and especially no clear explanation of the phenomenon was given. An analysis of accident databases existing in different countries is then presented. The figures are somewhat amazing and a yearly average exceeding 18 accidents was identified. Surprisingly three out of four accidents take place in the close vicinity of the ground where the recovery actions recommended in AC 90-95, the authoritative document about the topic, are not applicable. An explanation of the phenomenon is proposed, using the pedal curve as a tool. It allows understanding how and in what conditions unanticipated yaw occurs in the simplest case, hovering with wind. It also shows that recovery is affected by the modification of the pedal position at trim induced by the change in heading coming from the yaw rate, which makes the pilot feel the tail rotor to be ineffective. Accidents most probably occur because pilots do not use pedal corrections of sufficient amplitude during recovery. A more complex case is also analyzed, low speed turns to the right in ground reference with wind, as used during photographing or filming flights where unanticipated yaw often takes place. Such events are shown to occur when entering tailwind conditions, where the airspeed is reduced while a side wind component exists. A few myths grown on our poor understanding of the issue are then corrected, highlighting the unsafe way of flying helicopters with left wind, when performance is limited. The pedal curve provides a clear understanding of unanticipated yaw and gives an opportunity to solve that problem. This asks to build a unique, clear and consistent message toward pilots, appropriate to the low height conditions where the problem occurs, that shall be propagated by Authorities, Industry and Flight Schools
A coupled numerical/experimental study of flow separation suppression over a curved surface using fluidic oscillators
Get PDFFluidic oscillators, devices that generate sweeping jets when supplied with a pressurized fluid, have been used in a variety of flow control applications. The present investigations focus on understanding the physics and numerical prediction of these devices to control separation over a curved surface appropriate for rotorcraft applications. High-fidelity simulations and experimental data are employed to identify the mechanisms responsible for the control of separation. The model design includes an overhang at the interface between the actuators and the outer flow. At this interface, small scale spanwise vortices are shed in the streamwise direction, thereby enhancing wall-normal mixing. The spatial evolution of the sweeping jets give rise to large-scale structures between them causing spanwise mixing. In the mean sense, the jets lead to the formation of recirculation regions near the actuator exits inducing a deflection of the outer flow towards the wall. The simulations also examine the effects of fully resolving the interior of the oscillators, or using a boundary condition model, including turbulence. This boundary condition was found to be able to reproduce the correct physics of the flow control application, including mixing of the sweeping jets with the outer flow
Experimental Investigation of dynamic stall control on a rotor airfoil using unsteady plasma actuation
No full textExperimental study of the rotor airfoil’s dynamic stall control using plasma were carried out. The ability of unsteady plasma flow control was verified. It is found that only 20% duty cycle can achieve obvious control effects. Then the study of nondimensional frequency of the actuator was carried out. And the best control effect is observed when F+=1~2. Finally, according to experimental data analysis of the mechanism of plasma control, it is found that plasma excitation is mainly responsible after dynamic stall vortex’s shedding. Both steady excitation and unsteady excitation can significantly promote the recovery of the leading edge pressure gradient, and the effect of unsteady excitation is more effective
Helicopter main rotor dynamic properties identification using full scale ground and flight tests measurements
Get PDFThe one of the key challenges during new rotor blades design is the proper tuning of their dynamic properties. The specific design stages are defined as milestones and each particular stage needs specific methodology for a correct identification of the rotor dynamics. Every stage needs validated tools for a correct rotor characteristic frequencies prediction, which finally will be observed on the helicopter during normal helicopter operations. In this paper the review of estimation methodologies for the helicopter main rotor dynamic properties is presented with their application to new, improved W-3A Sokol helicopter MR blade set during design stage stand tests and during full scale helicopter on ground and in flight tests campaig